java - 当底层模型元素过多时，JComboBox的弹出菜单填充空白值

Question

首先，让我讨论一下数据模型：

我在树状对象模型中编译了 HL7 定义，具有 3 个级别：段、元素和子元素。段包含一个列表，元素包含一个列表。它们之间的关系有点像“大陆-乡村-城市”

其次，我创建了一个由 3 个组合框组成的 GUI 组件，每个组合框都有不同的 ComboBoxModel 访问前面描述的对象模型。

我在 ComboBoxModel 上应用了观察者模式，以便每个模型都知道它必须访问哪个 List<>：
- HL7DefinitionElementModel 观察 HL7DefinitionSegmentModel
- HL7DefinitionSubelementModel 观察 HL7DefinitionElementModel

当您在第一个组合框（Segment）中选择一个值时，HL7DefinitionElementModel 会更新并存储已选择的 Segment。数据已正确存储。第一个值正确显示在“关闭”的 JComboBox 中。但是当您尝试打开该 JComboBox（元素）时，如果元素的数量太大（大约 12 个），则调用的弹出菜单中的项目为空白。

我很乐意发布任何代码。如果需要，我可以发送整个项目，但无法将其简化为 SSCEE。

编辑：这是问题的说明
（http://imgur.com/AiJ8mu0）

编辑：这是一个 SSCCE：

import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
import java.util.Observable;
import java.util.Observer;
import java.util.Vector;

import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.event.ListDataListener;

public class Application extends JFrame {
  /**
   * 
   */
  private static final long serialVersionUID = -2517616123689799182L;

  public Application() {
    add(new HL7FieldSelector());
  }

  public static void main(String[] args) {
    Application a = new Application();
    a.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    a.setSize(new Dimension(400, 100));
    a.setVisible(true);
  }
}

class HL7FieldSelector extends JPanel implements ActionListener {
  /**
   * 
   */
  private static final long serialVersionUID = 6135074279269049773L;
  private JComboBox<HL7DefinitionSegment> segment;
  private JComboBox<HL7DefinitionElement> element;
  private HL7DefinitionSegmentModel segmentModel;
  private HL7DefinitionElementModel elementModel;

  public HL7FieldSelector() {
    segment = new JComboBox<HL7DefinitionSegment>();
    element = new JComboBox<HL7DefinitionElement>();

    segmentModel = new HL7DefinitionSegmentModel();
    elementModel = new HL7DefinitionElementModel();

    segmentModel.addObserver(elementModel);

    segment.setModel(segmentModel);
    element.setModel(elementModel);

    segment.addActionListener(this);
    element.addActionListener(this);

    initUI();
  }

  private void initUI() {
    this.setLayout(new GridLayout(1, 3));
    this.add(segment);
    this.add(element);
    this.setMinimumSize(new Dimension(500, 30));
    this.setPreferredSize(new Dimension(500, 30));
    this.setMaximumSize(new Dimension(500, 30));

    segment.setMaximumRowCount(10);
    element.setMaximumRowCount(10);
  }

  @Override
  public void actionPerformed(ActionEvent arg0) {
    setVisible(!isVisible());
    setVisible(!isVisible());
  }
}

class HL7DefinitionSegment implements Comparable<HL7DefinitionSegment>, Serializable {
  /**
   * 
   */
  private static final long serialVersionUID = 7922798285885405647L;
  private List<HL7DefinitionElement> elements;
  private String name;

  public HL7DefinitionSegment(String name, ArrayList<HL7DefinitionElement> elements) {
    this.name = name;
    this.elements = elements;
  }

  public HL7DefinitionSegment() {
    elements = new Vector<HL7DefinitionElement>();
  }

  public boolean addElement(HL7DefinitionElement element) {
    return elements.add(element);
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public List<HL7DefinitionElement> getElements() {
    return elements;
  }

  @Override
  public String toString() {
    return this.name;
  }

  @Override
  public int compareTo(HL7DefinitionSegment segment) {
    return name.compareTo(segment.getName());
  }
}

class HL7DefinitionElement implements Comparable<HL7DefinitionElement>, Serializable {
  /**
   * 
   */
  private static final long serialVersionUID = -5344721929162039227L;
  private String name;
  private String number;

  public HL7DefinitionElement(String name, String number) {
    this.name = name;
    this.number = number;
  }

  public HL7DefinitionElement() {
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public String getNumber() {
    return number;
  }

  public void setNumber(String number) {
    this.number = number;
  }

  @Override
  public String toString() {
    return number + " " + name;
  }

  @Override
  public int compareTo(HL7DefinitionElement element) {
    return number.compareTo(element.getNumber());
  }
}

class HL7DefinitionModel implements Serializable {
  /**
   * 
   */
  private static final long serialVersionUID = -8180582371776223436L;
  private List<HL7DefinitionSegment> segments;
  private static HL7DefinitionModel _instance;

  public HL7DefinitionModel(Vector<HL7DefinitionSegment> segments) {
    this.segments = segments;
  }

  public HL7DefinitionModel() {
    segments = new Vector<HL7DefinitionSegment>();
  }

  public boolean addSegment(HL7DefinitionSegment segment) {
    return segments.add(segment);
  }

  public List<HL7DefinitionSegment> getSegments() {
    return segments;
  }

  public void setSegments(List<HL7DefinitionSegment> segments) {
    this.segments = segments;
  }

  public static HL7DefinitionModel getInstance() {
    if (_instance == null) {
      _instance = new HL7DefinitionModel();

      for (int i = 0; i < 2; i++) {
        HL7DefinitionSegment s = new HL7DefinitionSegment();
        s.setName("SEG" + i);
        for (int j = 0; j < 20; j++) {
          s.addElement(new HL7DefinitionElement("" + j + j + j, "" + j));
        }
        _instance.addSegment(s);
      }
    }
    return _instance;
  }
}

class HL7DefinitionSegmentModel extends Observable implements javax.swing.ComboBoxModel<HL7DefinitionSegment> {
  private HL7DefinitionModel model;
  private HL7DefinitionSegment selectedItem;

  public HL7DefinitionSegmentModel() {
    this.model = HL7DefinitionModel.getInstance();
    this.selectedItem = getElementAt(0);
  }

  public void addListDataListener(ListDataListener l) {}
  public void removeListDataListener(ListDataListener l) {}

  @Override
  public HL7DefinitionSegment getElementAt(int index) {
    return model.getSegments().get(index);
  }

  @Override
  public int getSize() {
    return model.getSegments().size();
  }

  @Override
  public Object getSelectedItem() {
    return selectedItem;
  }

  @Override
  public void setSelectedItem(Object anItem) {
    this.selectedItem = (HL7DefinitionSegment) anItem;
    setChanged();
    notifyObservers();
  }
}

class HL7DefinitionElementModel extends Observable implements javax.swing.ComboBoxModel<HL7DefinitionElement>, Observer {
  private HL7DefinitionSegment segment;
  private HL7DefinitionElement selectedItem;

  public HL7DefinitionElementModel() {
    this.segment = null;
  }

  @Override
  public void update(Observable o, Object arg1) {
    if (o instanceof HL7DefinitionSegmentModel) {
      segment = (HL7DefinitionSegment) ((HL7DefinitionSegmentModel) o).getSelectedItem();
      setSelectedItem(getElementAt(0));
    }
  }

  public void addListDataListener(ListDataListener l) {}
  public void removeListDataListener(ListDataListener l) {}

  @Override
  public HL7DefinitionElement getElementAt(final int index) {
      return segment.getElements().get(index);
  }

  @Override
  public int getSize() {
    return (segment != null ? segment.getElements().size() : 0);
  }

  @Override
  public Object getSelectedItem() {
    return selectedItem;
  }

  @Override
  public void setSelectedItem(Object anItem) {
    this.selectedItem = (HL7DefinitionElement) anItem;
    setChanged();
    notifyObservers();
  }
}

要重现错误：
- 启动应用程序
- 单击第二个组合框
- 单击第一个，选择选项 2
- 单击第二个组合框
- 注意列表如何为空

如果您这样做：
- 启动应用程序
- 单击第一个组合框，选择选项 2
- 单击第二个组合框
- 注意列表的填充方式

编辑：这是我发现的其他内容：只有在弹出菜单中的元素多于初始“填充”时，弹出菜单才为空白。也就是说，如果组合框在第一次选择中填充了 20 个元素，那么在下一次选择中，如果元素数量为 21 或更多，则弹出菜单将为空白，如果元素数量为 20 或更少，则显示。